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Effect of surface modified fillers on the water absorption of a (RTV) silicone denture soft lining material
Journal of Dentistry, Vol. 24, No. 4, pp. 297-300,1996 Copyright 0 1996Publishedby Elsevier ScienceLtd. All rights reserved Printed in Great Britain 0300-5712/96 $15.00+ 0.00
0300-5712(95)00066-6
ELSEVIER
Effect of surface modified fillers on the water absorption of a (RTV) silicone denture soft lining material M. G. J. Waters*,
R. G. Jagger? and R. W. Winter’
“Department of Basic Dental Science and +Department of Restorative Dentistry, Chemistry and Applied Chemistry, University of Wales College, Cardie UK
Cardiff Dental School,
UK; and ‘School
of
ABSTRACT
Objectives:The purpose of the present study was to modify the filler content of an experimental room-temperaturevulcanizing (RTV) lining material that demonstratedhigh water sorption in order to produce a material with low sorption properties. Methods: Three new formulationswere prepared,each containing different hydrophobic silane-treated silicafillers. Water sorptionpropertiesfor specimensobtainedfrom theseformulationswere determined usingstandardexperimentaltechniques. Results:All formulations demonstratedgreatly reducedwater absorptionand low volume change. Conclusions:An experimentalRTV poly(dimethylsiloxane)denture soft lining material having low water sorption propertieshasbeen produced.Copyright 0 1996Publishedby Elsevier ScienceLtd. KEY WORDS:
Soft lining,
J. Dent
24: 297-300
1996;
Fillers,
Water
(Received
sorption 1 November
1994;
reviewed
INTRODUCTION High water sorption by denture soft lining materials is undesirable since absorption is associated with dimensional change. This may lead to stresses at the liner-denture base interface, resulting in reduced bond strengthl. Room-temperature vulcanizing (RTV) silicone soft lining materials have been associated with high water sorption valueslJ2. Braden and Wright2 theorized that the type of filler and the way that it is bonded to the polymer could be responsible for the high water absorption seen in the RTV silicone materials. The very low tensile strength of unfilled cross-linked poly(dimethylsiloxane)s makes the use of fillers essential. The addition of special silica fillers can increase the tensile strength of the cross-linked polymer by up to 40 times. The great increase in strength comes about because of the strong vulcanized polymer-filler bonding, which is a combination of both chemical and physical bonds. The amount of filler present in the material can vary from as little as 10% to as much as 40% w/w, depending on the type of filler and physical characteristics required for resultant rubber. Preliminary investigations demonstrated that an experimental RTV silicone soft lining material has
Correspondence shouldbe addressed to: Mr ment of Restorative Dentistry, Medicine, Dental School, Heath
R. G. Jagger, DepartUniversity of Wales College of Park, Cardiff CF4 4x/, UK.
23 February
1995;
accepted
28 March
1995)
favourable mechanical properties but high water absorption3. It was subsequently confirmed that the silica filler was the major cause of the water sorption4. When filler was removed completely from the polymer matrix, water sorption was minimal. A correlation was seen between the amount of filler and degree of water sorption. The original filler had been chosen because its surface had been treated to give it a hydrophobic nature; therefore it was somewhat surprising to see the experimental soft lining material exhibit such high water absorption. The present study investigates the effect of alternative silane-treated fillers on the sorption properties of the polymerized experimental material.
MATERIALS
AND METHODS
The constituents of the experimental soft lining material are shown in Table 1. All the silicone material was from the same batch. Addition of catalyst and cross-linker to the polymer base causes vulcanization to take place at room temperature by a polycondensation reaction with the elimination of alcohol. Four formulations were tested containing different surface-treated fillers: 1. Formulation A contained the original filler HDK 2000/4 (Wacker-Chemie, Munich, Germany). 2. Formulation B contained surface-treated filler R974 (Degussa, Frankfurt, Germany).
298
Table
1996; 24: No. 4
J. Dent.
i. Constituents
Polymer Catalyst Cross-linker Filler
of experimental
lining
materials
and D were desorbed in a desiccator at 37” C and again weighed at regular time intervals until equilibrium was achieved. Dimensional change during absorption was recorded by measuring the diameter and thickness of the specimens in millimetres at marked points on their surfaces, using a digital micrometer to an accuracy of 0.001 mm (Micro 2000, Moore and Wright, Sheffield, UK). Volume was calculated using the formula:
Hydroxy end-blocked poly(dimethylsiloxane) Dicarboxylate tin Mixture of alkoxy silanes Surface-treated silica filler
3. Formulation C contained surface-treated filler R805 (Degussa, Frankfurt, Germany). 4. Formulation D contained surface-treated filler 812 (Degussa, Frankfurt, Germany).
dxt
Twenty-nine per cent w/w of each filler was used in the respective formulations. The particle size and specific surface area of the fillers are given in Table II. Catalyst and cross-linking agents were added dropwise to the elastomer and mixing was carried out with a spatula on a Perspex block. Specimens 45 mm in diameter and 1 mm thick were prepared by packing the mixed dough into moulds which had been prepared by investing Perspex blanks of appropriate size, in a conventional dental flasking technique using a 50 : 50 stone : plaster mix. Specimens were allowed to cure for 24 h at room temperature. Following curing, the silicone rubber discs were removed, dried to a constant weight in a desiccator (to an accuracy of 0.001 g) and placed in water at 37” C. The discs were removed at intervals, excess water was blotted and discs were re-weighed as before. Five samples were tested for each formulation. This procedure was continued for specimens from groups A, B, C and D for 3 months. At 3 months, formulations B, C and D all showed very low water sorption. Because formulations B and C had shown unfavourable mixing characteristics it was decided to continue weighing only specimens of formulations A and D until equilibrium was achieved. After equilibrium was achieved, the specimens from these groups A Tab/e
II, Particle
size and specific
surface
area
Particlesize (nm)
BET surface fm”g-‘)
HDK 2000/4 R974 R805 R812
12 12 12 7
120+20 170+20 150t25 260 f 30
Tab/e 111. Water Formulation
A 6 C D
uptake
where r = radius; t = thickness. Percentage absorption and termined as follows:
Percentage
A-D
were
de-
(2)
1
% solubility = w, w- w, x 100 1 where W, = initial weight; W, = weight after absorption or desorption; W, = final weight after desiccation. A one-way analysis of variance (ANOVA) followed by multiple comparisons using the Bonferroni method were used to evaluate the difference between all formulations at 3 months (Table III). An unpaired two-tailed Student’s t-test was used to evaluate differences between formulations A and D at equilibrium (Table IV). A P value of < 0.05 was considered to be a significant difference.
RESULTS Table III shows the percentage of water uptake of the four formulations up to 3 months. This is illustrated graphically in Fig. 1. Equilibrium was achieved for formulation A at 22 months and for formulation D at 12 months. Table IV shows the mean percentage absorption, solubility and volume change of formulations A and D at equilibrium. All results are based on the mean values of the five specimens for each formulation. Formulation A had a significantly higher water uptake than formulations B, C and D at 3 months (P < 0.001). Of the new formulations B had a significantly higher uptake than C (P < 0.05) and D (P <
area
of formulations
solubility
% absorption = w, w- v? x 100
of fillers
Filler
(1)
at 3 months Percentage volume-change
uptake
Mean
s.d.
Mean
s.d.
18.13 3.71 1.80 0.72
1.30 0.40 0.11 0.05
20.21 2.13 0.94 0.82
1.61 0.14 0.06 0.05
Waters eta/,: Effect
Tab/e IV. Water equilibrium Formulation
A D
absorption,
Percentage absorption Mean 22.71 1.82
solubility
and
volume
change
of fillers
on soft
of formulations
lining
A and
s.d.
Percentage soiubility Mean
s.d.
Percentage vofume change s. d. Mean
1.25 0.11
0.99 0.75
0.02 0.03
25.76 1.13
O.OOl), whilst there was no significant difference between C and D. The final absorption figure at equilibrium for formulation D is very low and extremely significantly lower (P < 0.0001) than formulation A (Table N). The solubility and volume change are also low.
DISCUSSION The present investigation has determined water sorption values of a variety of formulations of an experimental denture soft lining material using standard experimental technqiues’,‘. The silica fillers used in this study were chosen because of their hydrophobic nature. Untreated silica fillers have moisture-attracting silanol groups on the surface. By reacting the silanol groups on the silica surface with dimethyldichlorosilane in a continuous technical process, hydrophobic silica can .be produced (F&. 2). Varying the silane treatment will produce a range of silica fillers with differing surface structures. The exact nature of the surface groups on the original filler (HDK 4000/4) is not known; however, the surface groups on the three new fillers are shown in Fig. 3. It is stated by the manufacturer that the original filler had a surface silanol group density in the range 0.5-2.3 SiOH nmm2. Information on the silanol group density of the new fillers was not given. However, it was stated that the surface silanols are so greatly reduced by the influence of the treatment that the silica is no longer capable of absorbing moisture. The reason why formulation A containing the original filler produces such high water absorption is uncer-
D at
1.63 0.09
tain. It is possible that the surface treatment did not sufficiently reduce the influence of the silanol groups on the silica surface. The nature of the filler polymer bonding may also have an effect on the rubber’s water absorption as suggested by Braden and Wright’. The accessibility of silanol groups on the surface of the silica will effect the nature of the bonding forces (hydrogen bonding and Van der Waal’s forces) prior to vulcanization. The treating of the silica to reduce silanol influence may therefore have an effect on this initial silica-polymer bonding. At 3 months, formulations B, C and D had greatly improved water uptake compared to formulation A which contained the original filler (Fig. 1). It was decided to continue to equilibirum only with formulation D containing filler R812, firstly because it showed the lowest water uptake at 3 months, and secondly because the viscosity of the other two formulations was so great that mixing of catalyst and cross-linker was difficult. The high viscocity was probably due to hydrogen bonding between filler particles when the formulations were left standing.
:: /A----y
A
Dimethylchlorosilane Continuous
Hydrophilic
process
silica filler
Fig. 2. Production dichlorosilane.
20 18
299
water sorption
Si
)
Hydrophobic
of hydrophobic
16
I3
silica
by reacting
silica filler with
dimethyl-
/ CH3
1
7
31
Y3
Days Fig. 1. Water uptake different surface-treated
of experimental silica fillers.
soft
lining
material
with
Filler R 974 Fig, 3. Surface
Filler R 805 structures
of fillers
R974,
Filler R 812 R805
and R812.
300
J. Dent. 1996; 24: No. 4
In this study an experimental silicone soft-lining material, which has previously been shown to have promising mechanical properties, has been successfully modified to reduce water absorption characteristics to a level comparable to that of poly(methylmethacrylate) denture base resin by the incorporation of an alternative surface-treated silica filler. The successful modification of the formulation to produce a silicone RTV soft lining material with low water absorption overcomes the inherent high water absorption characteristics that have been shown for such materials in the past’,‘. Further work is being undertaken to establish the mechanical and physical properties of the new formulation.
References 1. Wright PS. Soft lining materials: their status and prospects. J Dent 1976; 6: 247-256.
2. Braden M and Wright PS. Water absorption and water solubility of soft lining materials for acrylic dentures, J Dent Res 1986; 62: 764-768.
3. Waters MGJ and Jagger RG. Properties of an experimental silicone soft lining material. J Dent Res (abs) 1994; 73: 807. 4. Waters MGJ and Jagger RG. Water absorption of an (RTV) silicone denture soft lining material. J Dent 1996; 24: 105-108. 5. Stafford GD and Braden M. Water absorption of some denture base polymers. J Dent Res 1968; 47: 341. 6. No11 W. Chemistry and Technology of Silicones. London: Academic Press, 1968; pp 401-404.
0300-5712(95)00066-6
ELSEVIER
Effect of surface modified fillers on the water absorption of a (RTV) silicone denture soft lining material M. G. J. Waters*,
R. G. Jagger? and R. W. Winter’
“Department of Basic Dental Science and +Department of Restorative Dentistry, Chemistry and Applied Chemistry, University of Wales College, Cardie UK
Cardiff Dental School,
UK; and ‘School
of
ABSTRACT
Objectives:The purpose of the present study was to modify the filler content of an experimental room-temperaturevulcanizing (RTV) lining material that demonstratedhigh water sorption in order to produce a material with low sorption properties. Methods: Three new formulationswere prepared,each containing different hydrophobic silane-treated silicafillers. Water sorptionpropertiesfor specimensobtainedfrom theseformulationswere determined usingstandardexperimentaltechniques. Results:All formulations demonstratedgreatly reducedwater absorptionand low volume change. Conclusions:An experimentalRTV poly(dimethylsiloxane)denture soft lining material having low water sorption propertieshasbeen produced.Copyright 0 1996Publishedby Elsevier ScienceLtd. KEY WORDS:
Soft lining,
J. Dent
24: 297-300
1996;
Fillers,
Water
(Received
sorption 1 November
1994;
reviewed
INTRODUCTION High water sorption by denture soft lining materials is undesirable since absorption is associated with dimensional change. This may lead to stresses at the liner-denture base interface, resulting in reduced bond strengthl. Room-temperature vulcanizing (RTV) silicone soft lining materials have been associated with high water sorption valueslJ2. Braden and Wright2 theorized that the type of filler and the way that it is bonded to the polymer could be responsible for the high water absorption seen in the RTV silicone materials. The very low tensile strength of unfilled cross-linked poly(dimethylsiloxane)s makes the use of fillers essential. The addition of special silica fillers can increase the tensile strength of the cross-linked polymer by up to 40 times. The great increase in strength comes about because of the strong vulcanized polymer-filler bonding, which is a combination of both chemical and physical bonds. The amount of filler present in the material can vary from as little as 10% to as much as 40% w/w, depending on the type of filler and physical characteristics required for resultant rubber. Preliminary investigations demonstrated that an experimental RTV silicone soft lining material has
Correspondence shouldbe addressed to: Mr ment of Restorative Dentistry, Medicine, Dental School, Heath
R. G. Jagger, DepartUniversity of Wales College of Park, Cardiff CF4 4x/, UK.
23 February
1995;
accepted
28 March
1995)
favourable mechanical properties but high water absorption3. It was subsequently confirmed that the silica filler was the major cause of the water sorption4. When filler was removed completely from the polymer matrix, water sorption was minimal. A correlation was seen between the amount of filler and degree of water sorption. The original filler had been chosen because its surface had been treated to give it a hydrophobic nature; therefore it was somewhat surprising to see the experimental soft lining material exhibit such high water absorption. The present study investigates the effect of alternative silane-treated fillers on the sorption properties of the polymerized experimental material.
MATERIALS
AND METHODS
The constituents of the experimental soft lining material are shown in Table 1. All the silicone material was from the same batch. Addition of catalyst and cross-linker to the polymer base causes vulcanization to take place at room temperature by a polycondensation reaction with the elimination of alcohol. Four formulations were tested containing different surface-treated fillers: 1. Formulation A contained the original filler HDK 2000/4 (Wacker-Chemie, Munich, Germany). 2. Formulation B contained surface-treated filler R974 (Degussa, Frankfurt, Germany).
298
Table
1996; 24: No. 4
J. Dent.
i. Constituents
Polymer Catalyst Cross-linker Filler
of experimental
lining
materials
and D were desorbed in a desiccator at 37” C and again weighed at regular time intervals until equilibrium was achieved. Dimensional change during absorption was recorded by measuring the diameter and thickness of the specimens in millimetres at marked points on their surfaces, using a digital micrometer to an accuracy of 0.001 mm (Micro 2000, Moore and Wright, Sheffield, UK). Volume was calculated using the formula:
Hydroxy end-blocked poly(dimethylsiloxane) Dicarboxylate tin Mixture of alkoxy silanes Surface-treated silica filler
3. Formulation C contained surface-treated filler R805 (Degussa, Frankfurt, Germany). 4. Formulation D contained surface-treated filler 812 (Degussa, Frankfurt, Germany).
dxt
Twenty-nine per cent w/w of each filler was used in the respective formulations. The particle size and specific surface area of the fillers are given in Table II. Catalyst and cross-linking agents were added dropwise to the elastomer and mixing was carried out with a spatula on a Perspex block. Specimens 45 mm in diameter and 1 mm thick were prepared by packing the mixed dough into moulds which had been prepared by investing Perspex blanks of appropriate size, in a conventional dental flasking technique using a 50 : 50 stone : plaster mix. Specimens were allowed to cure for 24 h at room temperature. Following curing, the silicone rubber discs were removed, dried to a constant weight in a desiccator (to an accuracy of 0.001 g) and placed in water at 37” C. The discs were removed at intervals, excess water was blotted and discs were re-weighed as before. Five samples were tested for each formulation. This procedure was continued for specimens from groups A, B, C and D for 3 months. At 3 months, formulations B, C and D all showed very low water sorption. Because formulations B and C had shown unfavourable mixing characteristics it was decided to continue weighing only specimens of formulations A and D until equilibrium was achieved. After equilibrium was achieved, the specimens from these groups A Tab/e
II, Particle
size and specific
surface
area
Particlesize (nm)
BET surface fm”g-‘)
HDK 2000/4 R974 R805 R812
12 12 12 7
120+20 170+20 150t25 260 f 30
Tab/e 111. Water Formulation
A 6 C D
uptake
where r = radius; t = thickness. Percentage absorption and termined as follows:
Percentage
A-D
were
de-
(2)
1
% solubility = w, w- w, x 100 1 where W, = initial weight; W, = weight after absorption or desorption; W, = final weight after desiccation. A one-way analysis of variance (ANOVA) followed by multiple comparisons using the Bonferroni method were used to evaluate the difference between all formulations at 3 months (Table III). An unpaired two-tailed Student’s t-test was used to evaluate differences between formulations A and D at equilibrium (Table IV). A P value of < 0.05 was considered to be a significant difference.
RESULTS Table III shows the percentage of water uptake of the four formulations up to 3 months. This is illustrated graphically in Fig. 1. Equilibrium was achieved for formulation A at 22 months and for formulation D at 12 months. Table IV shows the mean percentage absorption, solubility and volume change of formulations A and D at equilibrium. All results are based on the mean values of the five specimens for each formulation. Formulation A had a significantly higher water uptake than formulations B, C and D at 3 months (P < 0.001). Of the new formulations B had a significantly higher uptake than C (P < 0.05) and D (P <
area
of formulations
solubility
% absorption = w, w- v? x 100
of fillers
Filler
(1)
at 3 months Percentage volume-change
uptake
Mean
s.d.
Mean
s.d.
18.13 3.71 1.80 0.72
1.30 0.40 0.11 0.05
20.21 2.13 0.94 0.82
1.61 0.14 0.06 0.05
Waters eta/,: Effect
Tab/e IV. Water equilibrium Formulation
A D
absorption,
Percentage absorption Mean 22.71 1.82
solubility
and
volume
change
of fillers
on soft
of formulations
lining
A and
s.d.
Percentage soiubility Mean
s.d.
Percentage vofume change s. d. Mean
1.25 0.11
0.99 0.75
0.02 0.03
25.76 1.13
O.OOl), whilst there was no significant difference between C and D. The final absorption figure at equilibrium for formulation D is very low and extremely significantly lower (P < 0.0001) than formulation A (Table N). The solubility and volume change are also low.
DISCUSSION The present investigation has determined water sorption values of a variety of formulations of an experimental denture soft lining material using standard experimental technqiues’,‘. The silica fillers used in this study were chosen because of their hydrophobic nature. Untreated silica fillers have moisture-attracting silanol groups on the surface. By reacting the silanol groups on the silica surface with dimethyldichlorosilane in a continuous technical process, hydrophobic silica can .be produced (F&. 2). Varying the silane treatment will produce a range of silica fillers with differing surface structures. The exact nature of the surface groups on the original filler (HDK 4000/4) is not known; however, the surface groups on the three new fillers are shown in Fig. 3. It is stated by the manufacturer that the original filler had a surface silanol group density in the range 0.5-2.3 SiOH nmm2. Information on the silanol group density of the new fillers was not given. However, it was stated that the surface silanols are so greatly reduced by the influence of the treatment that the silica is no longer capable of absorbing moisture. The reason why formulation A containing the original filler produces such high water absorption is uncer-
D at
1.63 0.09
tain. It is possible that the surface treatment did not sufficiently reduce the influence of the silanol groups on the silica surface. The nature of the filler polymer bonding may also have an effect on the rubber’s water absorption as suggested by Braden and Wright’. The accessibility of silanol groups on the surface of the silica will effect the nature of the bonding forces (hydrogen bonding and Van der Waal’s forces) prior to vulcanization. The treating of the silica to reduce silanol influence may therefore have an effect on this initial silica-polymer bonding. At 3 months, formulations B, C and D had greatly improved water uptake compared to formulation A which contained the original filler (Fig. 1). It was decided to continue to equilibirum only with formulation D containing filler R812, firstly because it showed the lowest water uptake at 3 months, and secondly because the viscosity of the other two formulations was so great that mixing of catalyst and cross-linker was difficult. The high viscocity was probably due to hydrogen bonding between filler particles when the formulations were left standing.
:: /A----y
A
Dimethylchlorosilane Continuous
Hydrophilic
process
silica filler
Fig. 2. Production dichlorosilane.
20 18
299
water sorption
Si
)
Hydrophobic
of hydrophobic
16
I3
silica
by reacting
silica filler with
dimethyl-
/ CH3
1
7
31
Y3
Days Fig. 1. Water uptake different surface-treated
of experimental silica fillers.
soft
lining
material
with
Filler R 974 Fig, 3. Surface
Filler R 805 structures
of fillers
R974,
Filler R 812 R805
and R812.
300
J. Dent. 1996; 24: No. 4
In this study an experimental silicone soft-lining material, which has previously been shown to have promising mechanical properties, has been successfully modified to reduce water absorption characteristics to a level comparable to that of poly(methylmethacrylate) denture base resin by the incorporation of an alternative surface-treated silica filler. The successful modification of the formulation to produce a silicone RTV soft lining material with low water absorption overcomes the inherent high water absorption characteristics that have been shown for such materials in the past’,‘. Further work is being undertaken to establish the mechanical and physical properties of the new formulation.
References 1. Wright PS. Soft lining materials: their status and prospects. J Dent 1976; 6: 247-256.
2. Braden M and Wright PS. Water absorption and water solubility of soft lining materials for acrylic dentures, J Dent Res 1986; 62: 764-768.
3. Waters MGJ and Jagger RG. Properties of an experimental silicone soft lining material. J Dent Res (abs) 1994; 73: 807. 4. Waters MGJ and Jagger RG. Water absorption of an (RTV) silicone denture soft lining material. J Dent 1996; 24: 105-108. 5. Stafford GD and Braden M. Water absorption of some denture base polymers. J Dent Res 1968; 47: 341. 6. No11 W. Chemistry and Technology of Silicones. London: Academic Press, 1968; pp 401-404.